The human spine provides a vast array of functions, many of which are mechanical in nature. The spine is constructed to allow nerves from the brain to pass to various portions of the middle and lower body. These nerves, typically called the spinal cord, are located in a region within the spine called the spinal canal. Various nerve bundles emerge from the spine at different locations along the lateral length of the spine. In a healthy spine, these nerves are protected from damage and/or undue pressure thereon by the structure of the spine itself.
The spine has a complex curvature made up of a plurality (24 in all) of individual vertebrae separated by intervertebral discs. These discs hold the vertebrae together in a flexible manner so as to allow a relative movement between the vertebrae from front to back and from side to side. This movement then allows the body to bend forward and backward, to twist from side to side, and to rotate about a vertical axis. Throughout this movement, when the spine is operating properly the nerves are maintained clear of the hard structure of the spine.
Over time, or because of accidents, the intervertebral discs loose height, become cracked, dehydrated, or herniated. The result is that the disc height is reduced leading to compression of the nerve bundles, causing pain and in some cases damage to the nerves.
Currently, there are many systems and methods at the disposal of a physician for reducing, or eliminating, the pain by minimizing the stress on the nerve bundles. In some instances, the existing disk is removed and an artificial disk is substituted therefore. In other instances, two or more vertebrae are fused together to prevent relative movement between the fused discs.
Often there is required a system and method for maintaining, or recreating, proper space for the nerve bundles that emerge from the spine at a certain location. In some cases a cage or bone graft is placed in the disc space to preserve, or restore, height and to cause fusion of the vertebral level. As an aid in stabilizing the vertebrae, one or more rods or braces are placed between the fused vertebrae with the purpose of the rods being to support the vertebrae, usually along the posterior of the spine while fusion takes place. These rods are often held in place by anchors which are fitted into the pedicle of the vertebrae. One type of anchor is a pedicle screw, and such screws come in a variety of lengths, diameters, and thread types.
One problem occurs during the implantation of these systems in the vertebrae of patients. It is desirable to conduct as much of the implantation procedure as possible underneath the skin to minimize the size of the incisions need to perform the procedure, thereby minimizing trauma to the patient and other complications that often results from large incisions. Cannulae are commonly used to provide a mechanism to deliver implants into patients and as a means to insert tools to the site of the implants. Simple cannulae, which are merely tubes passing through the skin of the patient to the surgery site, provide no mechanism to interact with the implants or the tools used in a procedure.
What is needed is an extension for use with a stabilization system for bony structures, the stabilization system using bone anchor assemblies and a connector to connect the bone anchor assemblies, where the extension provides a locking connection to the bone anchor assemblies such that the extension provides a secure pathway for tools and instruments to access the bone anchor assemblies and openings through which the connector can be set in place to connect the assemblies, where the extension may be used in any number of useful ways including compression, depth measurement, rod capture, spondylolisthesis, etc.